Re: Proper spacing for technique in public..
1) Get out of the seat. Since the person was really not interested in a seat but more in starting a confrontation, now they'll be in a less mobile position (sitting), or if they don't sit down, at least you'll be standing already, giving you a more mobile when they start physically attacking you.
On the other hand, sitting gives you a lower center, and you can pull someone down and put their head through the wall.
2) If you're confronted in parking lot, there's probably going to be one person in front of you who asks you some stupid question and another person coming up behind you. If it's not too late, you need to clock one of them in the face, slip past them and run.